Historically items were palletized or stacked manually by individuals who would stack items one on top of the other in a vertical fashion. This method was restricted however to a human's lifting range. Consequently, in some more recent stacking systems, the pallet and items to be stacked have been placed on a platform which descends into a "pit" as the items are stacked, while attempting to keep the top of the stack within the normal lifting range of a person. Although the pit loading method is not limited to a person's lifting range, it requires a scissor lift or the like to secure the top of the stack at the operator's level and to raise the stacked items from the pit for eventual transportation.
Another method for stacking items is to clamp or "squeeze" an item on opposite sides by lateral retention plates or pinch-bars. This type of gripping method uses powerful pinch-bars to lift an item to a predetermined height, thus allowing for the introduction of a succeeding item beneath the lifted item. After the succeeding item is positioned beneath the lifted item, the pinch-bars are lowered and released, thereby depositing the lifted item onto the succeeding item. Next, the pinch-bars descend and clamp the lower of the two stacked items. This process of clamping, lifting and stacking is repeated to form a pallet of stacked loads. This type of lifting method, however, is dependent upon significant lateral forces which may tend to bend or damage non-rigid items to be stacked. The bending creates an uneven surface on the clamped item which leads to the tilting of the stacked items.
Another conventional method in the stacking industry uses lifting forks to vertically raise successive items to be stacked thereafter. While the forks secure the first item at a predetermined stacking height, succeeding items are introduced beneath. Next, the forks are lowered and disengaged, thereby depositing the lifted item upon the succeeding item. This method has been adapted to include a pair of reciprocating lift forks which operate in an alternating fashion to lift and stack succeeding items. However, this fork-lifting cycle includes a substantial amount of dead time to include withdrawal, downward movement, approach and upward movement of the forks. Moreover, the alternating fork method requires constant monitoring to avoid a slight deviation in the timing sequence which may prevent effective stacking.
Yet another method for stacking items includes opposing support fingers engaged beneath a stacking position and guided along vertical rails. The finger supports project into an area beneath an item to be stacked. As the finger supports ascend vertically along the guide rails, a first item is lifted to a predetermined height. Thereafter, a succeeding item is introduced into the stacking position, the vertical rails are lowered and the top item is deposited upon the succeeding item as the finger supports are lowered. Downward movement of the finger supports causes the fingers to pivot in an upward direction and disengage from beneath the first item upon contact with the succeeding item. This method, however, tends to damage the surfaces of the stacked items when the fingers are sandwiched therebetween during their descent. Moreover, this finger support method fails to align the stacked items in a fashion suitable for eventual transport. Furthermore, the finger support lifting apparatus is typically limited to a carriage or item of predetermined dimensions.